710 b. shimek mingling of pleistocene formations 



Eecext Observations 

 illustrations of mingling 



During the past year two striking illustrations of such mingling were 

 observed by the writer, the one at Des Moines, 'Iowa, showing a transpo- 

 sition of yellow and gray loesses and a mixing of fossiliferous gray loess 

 with Wisconsin drift, and the other at Sioux Falls, South Dakota, show- 

 ing fossiliferous silt apparently between two drifts. 



THE DES MOINES SECTION 



Location. — This section was exposed for a short time in the excavation 

 made for the heating plant of the East Des Moines High School. The 

 locality lies near the border of the Des Moines lobe of the Wisconsin 

 drift sheetj and loess is here frequently found under Wisconsin till.^ 



In a portion of the' section under consideration, however, an unusual 

 mingling of Pleistocene materials was observed during a visit made by 

 the members of the staff of the Iowa Geological Survey, which was sub- 

 sequently more closely investigated by the Avriter. 



The loesses. — Throughout the region under consideration two loesses 

 appear in many places. The lower is the usual bluish gray post-Kansan 

 loess, which is here frequently fossiliferous, and the upper is tlie common 

 yellow later loess. 



The Wisco?isin dHft. — In the same region the Wisconsin till presents 

 a common yellow phase, throughout which small boulders and pebbles 

 are irregularly scattered. In the section under consideration a thickness 

 of 10 to 15 feet is exposed. 



Mingled loess and drift. — In the lower 6 to 8 feet of this section there 

 appear gray bands and irregular streaks and masses which can not be 

 distinguished from the gray post-Kansan fossiliferous loess, excepting 

 for the presence of occasional pebbles, and, moreover, in a portion of the 

 section a larger stratum of this gray material lies above yellow loess, thus 

 producing an apparent transposition of the loesses. 



A careful study of the section on the north and west sides of the exca- 

 vation, however, revealed the cause of this unusual relation. 



On the west side at the base the section (plate 41, figure 1) exposes 

 about 5 feet of yellow loess with few fossils (r, figure 1). Above this is 

 a stratum of gray loess 2 to 4 feet in thickness, with a few pebbles, and 

 scattered shells of terrestrial mollusks for the most part broken. The 

 remaining 10 to 12 feet of the section show Wisconsin drift, in the lower 



s See also McGee and Call on the Loss and associated deposits of Des Moines. Amer- 

 ican Journal of Science, 3d ser., vol, xxiv, 1S82, pp. 202-323. 



